The invention relates to the local oxidation of silicon (LOCOS) using silicon nitride masks, and to a method for controlling field inversion of LOCOS-produced oxide isolation regions.
LOCOS processing has gained wide acceptance in MOSFET technology. The advantages of LOCOS structures over planar structures include, insofar as thick oxide patterns are concerned, excellent definition of the thick oxide and smaller metalization steps (the LOCOS oxide beneath the metalization is at least partially recessed within the substrate).
Silicon nitride is widely used as an oxidation mask for LOCOS processing. Silicon nitride alone or, perhaps preferably, the composite silicon dioxide-silicon nitride is used as a thermal oxidation mask to form the recessed dioxide isolation islands which characterize LOCOS processing and structures. The recessed silicon dioxide not only provides electric isolation, but is also used to define adjacent substrate regions, such as active device regions. However, lateral growth of the oxide beneath the oxide-nitride composite mask can produce a laterally sloping oxide profile termed the "bird's beak", or a combination of this lateral beak and an upward-extending protuberance termed the "bird's head". See, e.g., Appels and Paffen, Phillips Research Reports, volume 26, number 3, June, 1971, pages 157-165 (hereafter, "Appels and Paffen"). These undesirable structures cause numerous problems.
First, the sloping "beak" may be undesirably removed during subsequent etching operations, thus exposing pn junctions, contact regions, and source and drain regions in the underlying substrate. Secondly, the sloping profile can cause ill-defined edges in adjacent regions and correspondingly ill-defined or varied electrical characteristics.
Referring to FIGS. 1-4, a third problem involves the formation of inversion-preventing impurity regions beneath the recessed oxide isolation islands. According to Appels and Paffen, the composite oxide-nitride mask can be used in a dual role--first as a diffusion mask and then as an oxidation mask--to form a diffusion region and the overlying isolation oxide island. Referring to the ideal situation depicted in FIG. 1, a buried impurity region 15I of substrate 11 is used to suppress unwanted inversion at the edges of the oxide island 16I. Ideally, the impurity region 15I encompasses the sidewalls 19I of the island 16I. Unfortunately, all too often this ideal situation does not occur.
Rather, and referring to FIG. 2, during formation of the oxide island and the underlying impurity layer, an impurity layer 14 is initially deposited within the substrate 11 using the composite oxide 12-nitride 13 LOCOS mask. As shown in FIG. 3, this impurity layer is expanded during oxidation and forms the inversion-suspending impurity layer 15. As is well known and as applicant has found during LOCOS processing, the impurity layer 15 typically does not expand laterally a sufficient distance to cover the sidewall 19 of the "bird's beak" 20. The exposed (inadequately implanted) substrate area increases as the thickness of the oxide mask 12 is increased and as the bird's beak is thus made increasingly long. The exposed substrate region beneath the side wall 19 is susceptible to being inverted by small voltages. The isolation oxide 16 circumscribes or borders active circuit areas such as transistors. At least some of the transistor channels are adjacent to and parallel to the exposed side walls. These exposed side walls provide low-threshold voltage, parasitic channels which are troublesome for memory as well as non-memory transistors. As shown in FIG. 11, for memory transistors the early turn-on effectively narrows the memory window between the erased (VTO) and written (VTI) states from w.sub.1 and w.sub.2.